Lore

When New Londo was flooded to contain their corrupted knights and their kings, the citizens of New Londo also fell victim to the flood. The ghosts of the citizens now haunt the ruins of their old city, inadvertently acting as guardians, keeping any intruder from getting close to the seal that keeps the Abyss and the Darkwraiths at bay.

Theories

Title of theory

Sources

Ghost BladeThe ghosts of New Londo clutch this wickedly sharp dagger with their bony hands and make mincemeat of the living.

Transient CurseLimb of the victim of a curse.
Temporary curse allows engagement with ghosts.
The only way to fight back against ghosts, who are cursed beings, is to become cursed oneself.
The safest method, however dreadful, is to cut off an arm of the dead.

Key to New Londo RuinsKey to the iron bars separating the ruins of New Londo and Drake Valley.
The ruins of New Londo were blocked off, for the cursed ghosts posed danger to life and spirit,
and the legends speak of a terrible Dark that was sealed away.

Key to the SealKey to the floodgates of New Londo, which seal away the Four Kings who fell to Dark.
The Sealers flooded New Londo to banish the Darkwraiths and the Four Kings.
The agonizing decision was made with the realization that countless lives, and the robust culture of the city, would be lost.
The victims now roam the ruins as ghosts.